How Can I Unpivot Columns in MySQL to Restructure My Data?

MySQL data reshaping: column expansion

When table data needs to be reshaped, column unpivoting becomes necessary. This technique converts columns into rows, providing a more flexible data representation. Although MySQL does not provide an explicit expansion function, we can achieve the same effect through clever query construction.

Method 1: Use UNION ALL to query

One way is to use a UNION ALL query, which appends the results of multiple SELECT statements together. In this example, we create a separate SELECT statement for each column that needs to be expanded:

<code class="language-sql">select id, 'a' col, a value
from yourtable
union all
select id, 'b' col, b value
from yourtable
union all
select id, 'c' col, c value
from yourtable</code>

This query builds a new table where each row represents a column value pair. The resulting table will have the following layout:

ID	col	value
1	a	a1
1	b	b1
1	c	c1
2	a	a2
2	b	b2
2	c	c2

Method 2: Use CROSS JOIN

Another approach is to use CROSS JOIN, which generates a Cartesian product of multiple tables. Combining yourtable with a table containing column names we can achieve the following:

<code class="language-sql">select t.id,
  c.col,
  case c.col
    when 'a' then a
    when 'b' then b
    when 'c' then c
  end as data
from yourtable t
cross join
(
  select 'a' as col
  union all select 'b'
  union all select 'c'
) c</code>

This query produces the same output as the UNION ALL query, giving us a flexible data representation for further analysis or data manipulation.

By implementing these unfolding techniques in MySQL, you can effectively extract meaningful insights from your data by reshaping it into a more easily navigable format.

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